Implantable prostheses are commonly used in medical applications. One of the more common prosthetic structures include tubular prostheses which may be used as vascular grafts to replace or repair damaged or diseased blood vessels. To maximize the effectiveness of such a prosthesis, it should be designed with characteristics which closely resemble that of the natural body lumen which it is repairing or replacing.
One form of a conventional tubular prosthesis specifically used for vascular grafts includes a textile tubular structure formed by weaving, knitting or braiding synthetic fibers into a tubular configuration. Tubular textile structures have the advantage of being naturally porous which allows desired tissue ingrowth and assimilation into the body. This porosity, which allows for ingrowth of surrounding tissue, must be balanced with fluid tightness so as to minimize leakage during the initial implantation stage.
Attempts to control the porosity of the graft while providing a sufficient fluid barrier have focused on increasing the thickness of the textile structure, providing a tighter stitch construction and including features such as velours to the graft structure. Further most textile grafts require the application of a biodegradable natural coating, such as collagen or gelatin in order to render the graft blood tight. While grafts formed in this manner overcome certain disadvantages inherent in attempts to balance porosity and fluid tightness, these textile prostheses may exhibit certain undesirable characteristics. These characteristics may include an undesirable increase in the thickness of the tubular structure, which makes implantation more difficult. These textile tubes may also be subject to kinking, bending, twisting or collapsing during handling. Moreover, application of a coating may render the grafts less desirable to handle from a tactility point of view.
It is also well known to form a prosthesis, especially a tubular graft, from polymers such as polytetrafluoroethylene (PTFE). A tubular graft may be formed by stretching and expanding PTFE into a structure referred to as expanded polytetrafluoroethylene (ePTFE). Tubes formed of ePTFE exhibit certain beneficial properties as compared with textile prostheses. The expanded PTFE tube has a unique structure defined by nodes interconnected by fibrils. The node and fibril structure defines micropores which facilitate a desired degree of tissue ingrowth while remaining substantially fluid-tight. Tubes of ePTFE may be formed to be exceptionally thin and yet exhibit the requisite strength necessary to serve in the repair or replacement of a body lumen. The thinness of the ePTFE tube facilitates ease of implantation and deployment with minimal adverse impact on the body.
One particular advantage of using ePTFE vascular grafts derives from its thinness. ePTFE vascular grafts possess a low profile because of its ultra-thin characteristics. This is particularly advantageous for implantable prostheses which are often delivered to the implantation site through blood vessels which are tortuous and narrow. The low profile facilitates delivery.
While exhibiting certain superior attributes, ePTFE tubes are not without certain disadvantages. Grafts formed of ePTFE tend to be relatively non-compliant as compared with textile grafts and natural vessels. Further, while exhibiting a high degree of tensile strength, ePTFE grafts are susceptible to tearing. Additionally, PTFE grafts lack the suture compliance of coated textile grafts. This may cause undesirable bleeding at the suture hole. Thus, the ePTFE grafts lack many of the advantageous properties of certain textile grafts.
It is also known that it is extremely difficult to join PTFE and ePTFE to other materials via adhesives or bonding agents due to its natural hydrophobic character. Wetting of the surface by the adhesive is necessary to achieve adhesive bonding, and PTFE and ePTFE are extremely difficult to wet without destroying the chemical properties of the polymer. Thus, heretofore, attempts to bond ePTFE to other dissimilar materials such as textiles, has been difficult.
It is also known to use vascular grafts in conjunction with support structures. Such support structures typically come in the form of stents, which are formed of metal or polymeric materials generally formed in a tubular structure and are used to hold a vein or artery open. Stents are well known in the art and may be self-expanding or radially expandable by balloon expansion. Examples of stent/graft configurations known in the art can be seen in U.S. Pat. Nos. 5,700,285; 5,749,880; and 5,123,917, each of which are herein incorporated by reference. It is advantageous to use stent/graft configurations because the stent provides and ensures the patency of the prosthesis, while the vascular graft provides biocompatible properties in a vessel more suitable for blood to flow through.
It is apparent that conventional textile prostheses as well as ePTFE prostheses have acknowledged advantages and disadvantages. Neither of the conventional prosthetic materials exhibits fully all of the benefits desirable for use as a vascular prosthesis.
It is therefore desirable to provide an implantable prosthesis, preferably in the form of a tubular vascular prosthesis, which achieves many of the above-stated benefits without the resultant disadvantages associated therewith. It is also desirable to provide an implantable multi-layered patch which also achieves the above-stated benefits without the disadvantages of similar conventional products.
It is also desirable to provide a prosthesis with the advantages of both textile and ePTFE artificial grafts, as well as the advantages of the graft being used in conjunction with a stent structure.